The present invention concerns a transmitting and receiving circuit for ultra-sonic flowmeters. The invention relates, particularly, to a transmitting and receiving circuit for an ultrasonic flowmeter, which includes a signal generator; and at least two circuit arrangements, each having a switching means and an ultrasonic transducer.
In such circuits it is known that measuring the very small time differences between the transmission of ultrasound upstream and downstream requires a very high degree of symmetry in the electronic circuits used for transmitting and receiving ultrasonic signals via the transducers, so that the group running time for signals in the electronic circuit itself is the same for both upstream and downstream signal transmission. It is also known that the impedances loading each transducer must be constant from the transmitting situation to the receiving situation. If this is the case, the sensor meets the condition of reciprocity. This means, among other things, that the upstream and down-stream transmission times of the sensor are equal when the medium stands still.
This problem is addressed in WO 94/17371, which describes the use of two identical transmitting and receiving circuits, one for each transducer, which include an amplifier with a first input terminal for connection to an ultrasonic flow transducer, a second input terminal for connection to a signal source, an output terminal for connection to a detection circuit and a feedback connection between the output terminal and the first input terminal. Each of the circuits is able to function as a transmitting circuit or a receiving circuit, and their mode of functioning is alternated by means of a switching arrangement, or by switching both circuits between the transmitting and receiving function to obtain the transmission of ultrasonic signals in both directions. However, variations caused by component tolerances, different temperature coefficients and the like between two such circuits, will still enable both fixed and temperature dependent group running time differences for signals in the electronic circuits.
DK 168 248 B1 discloses a system using separate transmitting and receiving circuits, thus running the risk of group running time differences of upstream and downstream signal transmission, as mentioned above.
The same applies for the system according to DK 166 974 B1, which consequently does not eliminate the problems of group running time differences. Further, these two systems are relatively complex, with several switches and amplifiers.
From DE 100 48 959 A1, filed by the applicant, a transmitting and receiving circuit for two ultrasonic flowmeters is known, which remedies the problems of variations between two such circuits caused by component tolerances, different temperature coefficients and the like, and which, at the same time, provides a simplification of the circuit used. This transmitting and receiving circuit includes two circuit arrangements, each consisting of a serial connection having an ultrasonic transducer and a switching means. The generator signal is led into a first input terminal (positive input terminal) of an amplifier, whereas the switching means are connected to a second input terminal (negative input terminal). In various embodiments, the circuit arrangement is either connected directly with a reference potential or with the amplifier output. This arrangement enables two-way transmission of ultrasonic signals with just one single transmitting and receiving circuit of the kind mentioned, because the switching means alternately connect one of the ultrasonic transducers functionally with the first input terminal. With the arrangement shown, component tolerances, different temperature coefficients, and the like, will be equal for the transmission of ultrasound in the two opposite directions, so that the group running times for signals in the electronic circuit itself are equal for the transmission of ultrasound in the two opposite directions. This applies for both fixed and variable group running time differences, the fixed differences being merely caused by component tolerances, the variable differences being caused by temperature dependent parameters. If the fixed group running time difference is eliminated to zero, zero point calibration/adjustment can be avoided, which reduces fault possibilities and saves time. In order to comply with, for example, authority approvals, the temperature dependent group running time difference must be kept within established limits. When transmitting ultrasonic signals in a first direction, the circuit is connected with the first ultrasonic transducer by means of the switching means, while a signal is transmitted to the transducer and converted to an ultrasonic signal. Subsequently, the circuit is switched to connection with the other ultrasonic transducer for receiving the signal, which will be generated when it receives the ultrasonic signal arriving from the first transducer. When transmitting the ultrasonic signal in the opposite direction, the circuit is first connected with the second transducer for transmission and subsequently with the first transducer for receiving.
With the embodiments described in DE 100 48 959 A1, it has turned out, however, that there is a problem with coupling of the signal from the generator to a receiving transducer and a problem with the decaying time of the transducers. Shortly after an ultrasonic transducer has acted as transmitter, the signal to the transducer is interrupted by a control device via the switching means mentioned above. However, the ultrasonic transducer continues to oscillate for some time after the switching, and this oscillation time depends on the design of the transducer. The transducer consists of a housing, typically of steel, in which the crystal is arranged, and the crystal transmits through a window in the housing. Generally, transducers with thick steel windows tend to oscillate for a longer time period than transducers with thinner windows because the thick window is adapted to the oscillation of the crystal, and the ringing has turned out to couple electrically to the receiving transducer, which causes a falsification of the measuring signal.
In principle, the problem can be solved by using only thin-walled transducers, but thick-walled transducers are preferred; because they are more robust towards eroding effects, the pressure of the medium (also vacuum), cavitation and fluid hammer. The use of thick-walled windows enables a selfbearing design and a design for higher pressures than possible with a relieved piezo ceramic used in the transducer.
DE 198 10 798 A1 teaches a solution for reducing the coupling of the generator signal and for reducing the effects of ringing from the ultra sonic transducer. A short circuit switch is inserted in the receiving circuit and in the transmitting circuit. The short circuit switches are controlled by a control arrangement which guides ringing signals and coupled generator signals to ground. However, the circuit uses separate transmitting- and receiving electronics and, therefore, has two different signal paths for transmitting and receiving signals. Thus, reciprocity is not given automatically, but must be made as described in DE 198 10 798 A1 by implementing an impedance matching network. This increases the complexity of the circuit and further a large number of short circuit-, transmitting- and receiving switches are necessary in order to obtain reciprocity and to reduce the couplings and ringing effects.